The group discovers that Jin is not among Nancy's witches. (Chapter 124)

Unlike Yuri's or Masamune's power, Jin's power is similar to Noa's.

Theory

Jin become a witch as soon as Urara Group's powers were erased, or not until before he kisses Midori?

Submission Power appears to work on Jin but is actually Placebo Effect: the tendency of any medication or treatment, even an inert or ineffective one, to exhibit results simply because the recipient believes that it will work.

Hypothesis 1: Jin was a witch before he became a member of Student Council.

Jin's only known desire is to be useful to Toranosuke. (Chapter 107 onward)

Could it be that Jin didn't gain power because his desire wasn't strong enough? A possibility.

A desire to be useful -> desire to be helpful -> wanting to help people -> a positive wish?

His desire is stronger because Toranosuke is oblivious to his attention and affection, as well as slight jealousy in bond between him and Ryu?

Hupothesis 2: Jin wasn't a witch before becoming a member of Student Council, but become one when his desire to be helpful to Toranosuke becomes strong enough. This can took place during Chapter 102 to Chapter 122.

Theory: Can one witch power be used on another witch?

While it is clarified that a witch cannot use her power on other witch due to the fact that they don't stack, Ryu's status as Witch Killer makes him an exception to this rule.

Ryu is not shown to, for example, be able to swap body with Nene, but he can charm her with her own power. The reason is because the momemt that he kiss, his current power gets overwritten.

Is it possible for an indirect kiss to render this rule invalid?

In a similar vein, the seventh witch is shown to be able to use her power on another witch, including another seventh witch as stated by Nancy when she uses her power on Rui Takuma. However, perhaps the gender of a witch is also the reason? Ryu uses Kotori (female)'s power on Jin (male), Nancy (female) uses her own power on Rui (male).

Similarly, Kotori used her power on Jin in chapter 111 and didn't found anything unusual, or she just didn't hear his thought?

Incidentally, Ryu cannot copy a witch's power through her medium. Could this hints something?

Theory: Witch Powers and Desire Types

It is confirmed that each of the witches and Witch Killers' power correspond to their own desires. Noa also speculated that those with power are usually born among students with problems.

Then, what did each of them wished for?

Urara hates her own life.

Nene wants to become a popular girl.

Meiko doesn't want to talk to people in general.

Tsubasa want to unite everyone in the basketball team.

Speculations: how about the rest?

Copy Power: to copy mean to be like someone else. Perhaps after Ryu's first year memories were erased, he want to be like someone?

Cut+Restore Power: to cut mean to take away something. Shinichi used to be a loner so jealousy is not to far from his personality at the time. Ushio obviously shows signs of jealousy.

Foresight Power: Based on Maria's personality, it is possible that she might wish to know what will happen in the future.

Traumatic Power: At the time, Noa wants to take revenge/blackmail on Seako, Keigo, and Ren, believing them to be one of her bullies. This topic is actually brought up by Midori when she suggests using Jin's power to take control of Toranosuke and the school.

Invisibility: to not wanting to be seen by others.

Amnesia: to erase unwanted memories.

Theory: Kissing and Using Power

Almost everyone with some kinds of power requires kissing, why?

Here is the list of those who requires kissing:

All the female witches except the seventh witches

All the Witch Killers

Jin Kurosaki

Those who doesn't require kissing to use power:

The seventh witches, however, both Rika and Nancy have to kiss to grant the witch at the ceremony.

Masamune Ichijo and Yuri Miura

When you think about kissing, the first thing that come to mind is the expression of love: friendship love (like Maria's kissing), or romantic love.